Ethernet devices having differential input/output (I/O) pins are typically connected to a communication medium (e.g., twisted-pair cables) through one or more transformers that provide some level of protection against undesirable voltage surges and/or electrostatic discharge (ESD) events. For example, FIG. 1 shows an Ethernet device 100 having an I/O interface 110, core circuitry 120, and a pair of differential I/O pins TRXP and TRXN. The I/O interface 110, which is typically an analog full-duplex interface, facilitates the exchange of signals between I/O pins TRXP/TRXN and core circuitry 120. The differential I/O pins TRXP/TRXN are connected via differential signal lines 31-32 to a transformer 20, which in turn is connected to a communication cable 10 (e.g., a twisted pair cable). In addition to transforming the voltages of signals provided on cable 10 to voltages more suitable for use by device 100, transformer 20 may protect circuitry within device 100 against surge energy emanating from cable 10.
More specifically, the surge energy emanating from cable 10 may be as high as several thousand volts, with cable 10 typically having a low output impedance (e.g., tens of ohms). Although some of this surge energy may be blocked by transformer 20, remnants of this surge energy are typically transferred to device 100 in the form of undesirable common-mode surges and undesirable differential mode surges. Common-mode surges typically refer to energy surges that elevate voltage levels on both differential I/O pins TRXP/TRXN at the same time, while differential mode surges typically refer to energy surges that elevate the voltage difference between the differential I/O pins TRXP/TRXN.
Industry standards typically specify Ethernet device 100 to have about 5 kV of common-mode surge tolerance. While transformer 20 is effective in blocking most of the common-mode energy surges, transformer 20 is less able to block the differential mode energy surges. Thus, it is usually much harder to protect device 100 from differential mode energy surges than from common-mode energy surges. Indeed, because the transformer 20 is typically designed to block common-mode signals and to pass differential signals, transformer 20 typically provides very limited protection against differential mode energy surges.
Thus, it would be desirable to provide increased protection for Ethernet devices against undesirable energy surges in a manner that does not adversely affect performance of the device.